Overpressure seal for differential pressure responsive device



J. E. M EvoY Re. 26,813

OVERPRESSURE SEAL FOR DIFFERENTIAL PRESSURE RBSPONSIVE DEVICE March 10,1970 Original Filed Nov. 7. 1966 I N VEN TOR. James 5. M:- [my UnitedStates Patent 26,813 OVERPRESSURE SEAL FOR DIFFERENTIAL PRESSURERESPONSIVE DEVICE James E. McEvoy, Painesville, Ohio, assignor to BaileyMeter Company, a corporation of Delaware Original No. 3,400,638, datedSept. 10, 1968, Ser. No. 592,528, Nov. 7, 1966. Application for reissueOct. 30, 1968, Ser. No. 791,820

Int. Cl. FlSb 15/22; F01b 19/00; G01] 7/06 US. C]. 92-10 Claims Matterenclosed in heavy brackets appears in the original patent but forms nopart of this reissue specification; matter printed in italics indicatesthe additions made by reissue.

ABSTRACT OF THE DISCLOSURE A dilferential pressure responsive devicehaving two bellows attached to a partition wall and enclosing a body ofliquid. The partition wall has a passageway connecting the interiors ofthe bellows and is provided with a free floating valve acting as anoverpressure seal.

This invention relates to a differential pressure responsive device ofthe opposed bellows operated type and more specifically to anoverpressure seal for such differential responsive devices.

The conventional bellows capsule is characterized by a central partitionhaving two opposed bellows mounted thereon. These bellows, eachconsisting of many convolutions, have their interiors in communicationwith each other and are filled with an incompressible fluid. The outerends of the bellows are joined internally at their centers by aconnecting stem passing freely through the central partition. Valvemembers are mounted on the connecting stem in each chamber and matingseats are provided in the respective sides of the central partition.Under normal operating conditions, the valves are open to permit freeflow of the incompressible fluid between the bellow chambers as theouter ends of the bellows move in unison in response to changes indifferential pressure. However, when either bellows is subjected tooverpressure, the stem movement causes the respective valve member toseat on the central partition thereby isolating the bellow chambers andpreventing further flow of the incompressible fluid. The isolation ofthe bellows, which results in trapping the fluids in each bellows,thereby maintaining a constant volume, protects the bellows fromsustaining damage due to overpressure.

In this double bellows liquid fill arrangement for overpressureprotection considerable motion is required for the valve members to seatproperly. This necessity for considerable stem motion requires sensingbellows of many convolutions. A seal that requires considerable stemmovement and thus a bellows of many convolutions results in undesirableincrease in capsule size and cost as well as undesirable increase ininternal volume. Furthermore, a sensing bellows consisting of manyconvolutions increases the likelihood of sediment or particlesdepositing on the surface of the bellows. The presence of this foreignmatter may partially restrict the movement of the bellows in response toa ditferential pressure change and thereby introduce a change or errorin the relationship between pressure difi'erential and bellows movement,or the weight thereof may cause an appreciable error in the calibrationof the unit.

The requirement for many convolutions necessitates a large volume ofincompressible fluid. Such fluid is sensitive to fluctuations intemperature as manifested by an increase in volume due to an increase intemperature.

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Fluid expansion causes a build-up of internal pressure resulting inmovement of the free outer ends of the bellows. In most two-chamberdesigns, the force beam, which transmits the bellow motion to anexternal transmitter generating an electric or pneumatic output signal,is connected to the free outer end of one of the bellows. Therefore, theinternal pressure build-up due to increase in temperature will causemovement of the force beam. Such a movement is not the result of achange in pressure dilferential and results in the signal generated bythe transmitter being in error.

Differential pressure responsive devices of the type here underdiscussion are quite commonly used in a force-balance transmitterwherein the force produced by the differential pressure acting on thebellows capsule is opposed by an equal and opposite force appliedexternally to the pressure housing. As a matter of fact, I will describemy invention for use in a transmitter of this type. In such atransmitter, the bellows motion required during normal operation isminute as it is essentially a null-balance device. Therefore, anyappreciable motion of the bellows required to operate the overpressureseal usually requires a break-away" linkage or some other form of lostmotion protective mechanism to maintain the null-balance mechanismwithin operating range, hence adding considerably to the cost andcomplication of the transmitter.

One object of the present invention is to provide an improved bellowsoperated pressure responsive device which is, for example, responsive tothe differential between the pressures upstream and downstream withrespect to an orifice plate, nozzle or Venturi tube.

Another object of my invention is to reduce the required number ofbellows convolutions.

Another object of my invention is to provide improved overpressureprotection of the sensing capsules.

Still another object of my invention is to reduce the effect on outputsignal caused by changes in temperature of the incompressible fluid withwhich the bellows capsules is filled.

A further object of my invention is to minimize the deleterious effectof the deposition of sediment on the sensing capsules.

My improved differential pressure responsive device consists of twobellows capsules sealed to opposite sides of a back-up plate. Thecapsules are mounted in opposition to one another with their remote endsfree to respond to pressure changes. The capsules are in communicationinternally by means of a passage through the back-up plate and theinternal volume created by the [essembly] assembly of capsules andback-up plate is filled with an incompressible fluid.

The back-up plate establishes two separate chambers in the pressurehousing and the free-moving end of each capsule is sensitve to thepressure existing in its respective chamber. Since the capsules aremounted in opposition to one another, the net bellows displacement.representing the difierential pressure, causes a movement in a forcebeam which is attached to the free-moving end of one capsule. Themovement of the force beam creates an unbalance in an associatedtransmitter which produces an output signal proportional to thedifferent pressure.

A push rod is attached internally to the center of the free-moving endof each bellows capsule and is of such length as to penetrate slightly ashoulder at the entrance to the passage in the back-up plate.

The shoulders at either end of the passage in the backup plate serve toretain a sealing ball which is free floating in a chamber within thebackup plate. During normal operating conditions, the ball and rodarrangement permits fluid to flow from one capsule bellows to the otheras the capsules respond to varying pressure differential. However,during excess pressure periods, the inner surface of the shoulders actas valve seats for the ball which cooperate to form a novel means forprotecting the bel lovvs capsules from sustaining damage due to anoverpressure condition.

This overpressure protection is accomplished by the bellows, under theexcess pressure condition, forcing the respective push rod against thescaling ball, the movement of the push rod seating the sealing ball onthe shoulder seat in the back-up plate. After the ball is seated, thuspreventing any substantial further flow of fluid into the lower pressurebellows capsule, any further pressure excrted against the bellowscapsule creates a slight movement of the bellows, but the decrease involume produces a relatively large displacement of hydraulic fluid whichis available for additionally forcing the ball against the seat therebyinsuring a positive leak-proof seal and protection from bellows damage.

Referring to the drawing, I show my novel differential pressureresponsive device arranged to operate a pneumatic force balancetransmitter. The transmitter, responding to the pressure differentialsensed by the bellows capsule transmits a signal to an indicator whichdisplays a visual indication of the sensed process pressuredifferential.

While the bellows capsule differential pressure responsive device andother features of the present invention will be discussed in connectionwith a pneumatic force balance transmitter, it will be understood thatit is equally applicable to an electric transmitter and that the signaloutput of the transmitter may, in addition to operating an indicator,operate a recorder, integrator, control device or the like.

In the general operation of the system of the drawing, l

a pressure differential of a process fluid is measured as it wouldappear, for example, across an orifice, flow noz. zle or Venturi tube.The pressure differential is conveyed through pressure ports 29 and 31to sensing capsules 5 and 7 respectively located within pressure housing3. Movement of the sensing capsules 5 and 7, due to variations in thepressure differential existing between pressure chambers and 27, isconveyed to one end of a force beam 21 by way of connecting strap 19.The force beam 21 passes through a sealing diaphragm 23 which, inaddition to sealing the chamber 27, acts as a fulcrum for the force beam21. The opposite end of the force beam 21 carries a vane of avane-nozzle fluid pressure couple. The nozzle 37 of this couple isrigidly fastened to the transmitter housing and a pipe extends from thenozzle to a booster 41.

The vane 35 is maintained in spaced relationship with the nozzle 37 andany change in this relationship causes the output of the booster tochange accordingly. The output pressure is fed back to a bellows 39 forrestoring the position of the force beam 21 and re-establishing theoriginal vanenozzle relationship. The output pressure is alsotransmitted to an indicater 43 representing a typical receiving device.

The single convolution bellows capsules 5 and 7 are scaled to eitherside of a back-up plate 9. The capsules 5 and 7 are mounted opposingeach other and are maintained in communication with each other duringnormal operating conditions by way of a passage 14 in the backup plate9. In fabricating the bellows capsule, the internal volume of thebellows capsules 5 and 7 and the back-up plate passage 14 is subjectedto a hard vacuum and then filled with an incompressible fluid 11 througha fill port 28. Having completely filled the internal volume, a fillport seal cap 30 is inserted. The process fluid fills the pressurechambers 25 and 27 through pressure ports 29 and 31 respectively andacts on the surfaces of the bellows capsules 5 and 7. The incompressiblefluid 11 insures positive, instantaneous transmission of motion of onebellows capsule to the other in response to it change in pressuredifferential.

The overpressure protective scheme incorporated in the differentialpressure responsive device to prevent overpressure damage to the bellowscapsules consists of two bcl lows push rods and a free floating ballseal. As shown in the drawing, push rods 13 and 15 are fixed to thecenters of the free-moving ends of the bellows capsules 5 and 7. Thesize of the push rods 13 and 15 is such as to permit them to penetrateshoulders 16 and 18 of the back-up plate passage 14 during normaloperation. Suflicient clearance is maintained between the push rods 13and 15 and the shoulders of the back-up plate 9 to permit theincompressible fluid 11 to press freely between bellows capsules 5 and7.

Within the back-up plate 9 is a center chamber 20 which, in conjunctionwith the shoulders 16 and 18, retain a free floating compressiblesealing ball 17. The clearance established between the center chamber 20and the sealing ball 17 permits flow of the incompressible fluid 11between bellows capsules 5 and 7 during normal differential pressures.The internal surfaces of the back-up plate shoulders 16 and 18 serve asvalve seats for the sealing ball during periods of excess overpressure.

In order to more clearly understand the operation of the overpressureprotective scheme, a hypothetical situation will be assumed. If thepressure in chamber 25 exceeds the pressure in chamber 27 by an amountgreater than the calibrated full range value, the movement of bellowscapsule 5 positions push rod 13 against the sealing ball 17 until thesealing ball seats on shoulder 16. The sealing ball 17 having seated onthe shoulder 16, the flow of incompressible fluid from bellows capsule 5to bellows capsule 7 is sealed off thereby isolating the bellowscapsules and trapping the incompressible fluid. The isolation of thebellows capsules 5 and 7 prevents bellows damage which would result ifbellows capsule 5 were permitted to collapse completely thereby forcingbellows capsule 7 to accept an excessive amount of displacedincompressible fluid.

1f the seating of sealing ball 17, by the operation so far described, isnot perfect, resulting in a leakage of fluid from capsule S to capsule7, the pressure in chamber 25 [of the capsule 5] will cause a furtherdecrease in internal volume of the capsule 5 which will cause adisplacement of fluid into the chamber 20. The fluid so displacedfurther compresses the sealing ball 17 against shoulder 16. Because ofthe difference in areas of the capsule S and chamber 20, a relativelyminute movement of the free end of capsule 5 will displace sufficientfluid into chamber 20 to cause an appreciable compression of the ball17, if required, to seal preferably the passage between capsule 5 andcapsule 7.

To produce a comparable compression of the ball 17 by means ofconventional designs presently available, which mechanically attach themovable member of the sealing valve connecting stern between thebellows, the bellows motion would *be considerably greater, as the ballmovement would be exactly equal to the movement of the free end of thebellows.

The foregoing discussion would apply equally to a reversal of theoverpressure condition, bearing in mind that the system reactions wouldalso be reversed.

[Various changes may be made in the details of construction withoutdeparting from the spirit and scope of my invention as defined by theappended claims] What I claim as new and desire to secure by LettersPatent of the United States is:

1. A diflerential pressure responsive device comprising a housing, apartition dividing said housing into two compartments, two opposedbellows of relatively large area having their adjacent ends sealed toopposite sides of said partition and their opposite ends movable inaccordance with changes in the difference in pressure between saidcompartments, a passageway through said partition having a relativelysmall area with respect to the effective areas of said bellows. saidbellows and said passageway being filled with an incompressible liquid,

and a free floating valve means for sealing said passageway upon anexcessive difference in pressure between said compartments operated bythe displacement of hydraulic fluid from the bellows compressed by saidexcessive difference in pressure into said passageway, the diiferencebetween the effective area of said last said bellows and said passagewaymaking available a relatively large volume of incompressible fluid uponcompression of said last named bellows for displacement into saidpassageway for operating said valve means to effectively seal saidpassageway.

2. A differential pressure responsive device according to claim 1wherein said passageway includes a chamber and said valve means seatsagainst an internal wall of said chamber to seal said passageway.

3. A differential pressure responsive device according to claim 2wherein the movable end of each of said bellows is provided with a pushrod for initially moving said valve means against the wall of saidchamber upon the difference in pressure between said compartmentsbecoming excessive.

4. A differential pressure responsive device according to claim 3wherein said valve means is a ball confined and freely movable in saidchamber.

5. A differential pressure device according to claim 4 wherein said ballis made of elastic material deformable against a wall of the chamber bythe displacement of hydraulic fluid from a bellows into said chamberupon the difference in pressure between said chambers becomingexcessive.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

MARTIN P. SCHWADRON, Primary Examiner I. C. COHEN, Assistant ExaminerUS. Cl. X.R.

